Book cover for Astronomy

Astronomy

Andrew Fraknoi, David Morrison, Sidney C. Wolff

ISBN #9781938168284

1st Edition

1,010 Questions

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Homework Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

This chapter section explains how stars evolve from their early life on the main sequence to later stages as red giants. It emphasizes that the evolution is determined by changes in core composition resulting from nuclear fusion, and that a star’s mass plays a key role in its fusion processes and evolution rate. H–R diagrams, especially the main-sequence turnoff points, are powerful tools for determining the ages of star clusters, thereby providing strong evidence to support stellar evolution theories.

Learning Objectives

1

Describe the evolutionary process of stars from the zero-age main sequence to the red giant stage.

2

Explain how changes in core composition influence a star's departure from the main sequence.

3

Analyze the impact of a star’s mass on its evolution and fusion processes.

4

Interpret H–R diagrams and main-sequence turnoff points to determine the ages of star clusters.

Key Concepts

CONCEPT

DEFINITION

Zero-age Main Sequence (ZAMS)

The point at which a star begins hydrogen fusion in its core, marking the start of its life on the main sequence.

Red Giant

A late stage in a star’s life where hydrogen fusion occurs in a surrounding shell and the outer layers expand significantly.

Core Composition

The mix of elements in the star’s core, primarily hydrogen initially, which changes as hydrogen is fused into helium and other heavier elements in more massive stars.

H–R Diagram

A graphical representation of stars plotting their luminosity against temperature, used to study stellar evolution and determine ages of star clusters.

Main-Sequence Turnoff Point

The point on the H–R diagram where stars begin to leave the main sequence, indicating the age of the star cluster.

Example Problems

Example 1

Compare the following stages in the lives of a human being and a star: prenatal, birth, adolescence/ adulthood, middle age, old age, and death. What does a star with the mass of our Sun do in each of these stages?

Example 2

What is the first event that happens to a star with roughly the mass of our Sun that exhausts the hydrogen in its core and stops the generation of energy by the nuclear fusion of hydrogen to helium? Describe the sequence of events that the star undergoes.

Example 3

Astronomers find that $90 \%$ of the stars observed in the sky are on the main sequence of an $\mathrm{H}$ -R diagram; why does this make sense? Why are there far fewer stars in the giant and supergiant region?

Example 4

Astronomers find that $90 \%$ of the stars observed in the sky are on the main sequence of an $\mathrm{H}$ -R diagram; why does this make sense? Why are there far fewer stars in the giant and supergiant region?

Example 5

Describe the evolution of a star with a mass similar to that of the Sun, from just after it first becomes a red giant to the time it exhausts the last type of fuel its core is capable of fusing.
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Step-by-Step Explanations

QUESTION

How does a star evolve from the main sequence phase to the red giant phase?

STEP-BY-STEP ANSWER:

Step 1: Begin with a star on the zero-age main sequence where hydrogen fusion in the core produces helium.
Step 2: As hydrogen in the core is depleted, the balance between the gravitational collapse and thermal pressure is disrupted.
Step 3: The decrease in core pressure leads to contraction of the core and an increase in temperature and density.
Step 4: The star's outer layers expand due to increased energy release from surrounding hydrogen shell fusion, transitioning the star into the red giant phase.
Step 5: For stars of higher mass, additional fusion processes may occur, further influencing evolutionary steps beyond the red giant stage.
Final Answer: A star evolves from the main sequence to the red giant stage as its core runs out of hydrogen, contracts, and heats up, causing the outer layers to expand and cool, and this process is influenced by the star’s mass and the subsequent changes in core composition.

Evolution from the Main Sequence to Red Giant Stage

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Common Mistakes

  • Confusing the zero-age main sequence with later stages of evolution.
  • Underestimating the impact of a star's mass on its rate of evolution and fusion processes.
  • Assuming that all stars follow the same evolutionary path regardless of differences in core composition changes.
  • Misinterpreting H–R diagrams without considering the significance of the main-sequence turnoff point.